The notebook can be downloaded at: http://ajp.aapt.org/resource/1/ajpias/v75/i4/p309_s1?isAuthorized=no

Description

Measuring the gravitational attraction between small ev eryday objects is difficult due to the extreme weakness of the gravitational force. The Cavendish balance is a torsion bal ance that is sufficiently sensitive to quantitatively measure the attraction. Gravitational forces between lead spheres are used to rotate the balance between two equilibrium positions. When the outer masses are in one orientation, it leads to an equilibrium position for the interior masses where the torques resulting from the gravitational and torsion forces are equal and opposite. Rotating the exterior masses to another orientation changes the torques on the balance, resulting in oscillatory motion to a new equilibrium position.

Even by utilizing the high sensitivity of a torsion balance, the gravitational attraction between the masses can rotate the torsion pendulum only through very small angles. An optical lever is used to turn this relatively small rotation of the bal ance into a larger linear translation by reflecting a laser off of a concave mirror mounted on the torsion wire. Position mea surements of the reflected laser then communicate the mo tion of the balance itself.